This project proposes to synthesize a number of ATP analogs in which the tripolyphosphate end of the molecule is modified. These analogs will be tested to see if they are substrates of myosin containing systems. If so, the kinetics of hydrolysis including the pH dependence, the effect of divalent metal ions, the effect of temperature and ionic strength will be determined and compared with ATP. If the analogs are not substrates, then binding and possible inhibition of ATP hydrolysis will be studied and the parameters of the system defined. The analogs will also be studied to determine their effects on the more complex actomyosin and muscle fibril systems in an effort to ascertain the exact substrate requirements for contraction. The analogs will be tested for their ability to exchange phosphate oxygens with water or to serve as nucleotide co-factors for inorganic phosphate-water exchange since these reactions have unique characteristics for contractile proteins. In particular, analogs capable of covalently labelling myosin, actin or other contractile proteins will be sought. Where appropriate, other ATP splitting enzymes will be tested with these affinity labelling compounds to compare their active sites with myosin and related proteins.